The Great Migration is one of the most awe-inspiring events in nature. Each July millions of ungainly, loping wildebeest migrate north from the Serengeti plains in search of fresh pasture in the Masai Mara. And because there is safety in numbers, along come thousands of zebra too, and antelopes of almost infinite variety: gazelles, topi, hartebeest, impala and elands. It is an awesome sight to come over the crest of an escarpment and see nothing but wildebeest as far as the horizon. Even more spectacular, if you’re lucky enough, is the sight of a river crossing. Lemming-like the animals gather by the thousands at the river’s edge, alternately turning back and peering into the water as if in search of crocodiles or worse, lions. It looks like nothing will happen til without warning one brave creature leaps into the water and suddenly all you can hear are the thunder of hooves as en masse the herd charges across the river. It’s raw and ungraceful, this desperate race to the other side; but the return to dry land heralds, momentarily anyway, a respite from death and a fresh pasture on which to graze.